Thermodynamics studies the relations between heat, energy (e.g., mechanical, electrical and chemical energy) and other quantities such as entropy and information. The first law of thermodynamics is a restatement of the principle of conservation of energy. The second law of thermodynamics states that heat can only spontaneously flow from a hotter to a cooler body or, equivalently, that the entropy of a closed system can only increase. For large systems, these laws can be expressed in terms of deterministic mathematical relations between the above-mentioned quantities. However, when dealing with microscopic systems, they unveil their true statistical nature: due to the presence of thermal noise, large fluctuations can occur leading, e.g., to an occasional decrease of entropy, as shown in Fig. 21.1. This Chapter will review how optical tweezers have been instrumental in studying the thermodynamics of microscopic systems.
21.1 Violation of the second law
21.2 The Jarzynski equality
21.3 Information-to-energy conversion
21.4 Micrometre-sized heat engine
21.5 Further reading